Are you blind? Go to coinmarketcap.com and see what is the total crytpo volume. BTC and LTC compromise more than 95% of the daily volume. Alts are nothing, compared to the Big 2.
LTC is an Alt, the OP is completely right there is absolutely no doubt BTC would be worth more if no alt existed. The total money going into Alts is more than their market cap as millions are spent in investments/ASICs, development hours!, and startups.
Could Alts delay or even prevent another BTC bubble, possibly yes, but then you better keep an eye out because there will be a gorilla. I still think it is Bitcoin, Alts are just a distraction with a benefit, which is to prove out important technologies that Bitcoin needs to adopt. I think Bitcoin can adapt to pressures of other coins if the pressure is high enough. So far the pressure hasn't been enough.
Why add speed if the best speed can get you is Litecoin, why add anonymity if the best anonymity can get you is DarkCoin, et cetera...